This invention relates generally to packet-switching systems, such as asynchronous transfer mode (ATM) systems, and specifically to transmission-delay variations in such systems.
Today""s business communications environment consists of two separate network infrastructures: a voice network (such as a private branch exchange (PBX)) characterized by real-time, high reliability, constant bit-rate (CBR) connections; and a data network (such as a packet network) characterized by high-bandwidth variable bit-rate (VBR) connections. Business needs for simplified maintenance, management, and access to information on diverse networks are forcing the convergence of these networks along with a new class of real-time multimedia networks. Asynchronous transfer mode (ATM) provides a single infrastructure that cost-effectively and flexibly handles both switching and transmission for the traffic types mentioned above (voice, video, and data) for both local-area networks and wide-area networks. The evolving network convergence requires the adaptation of the legacy PBX voice traffic to ATM. Voice telephony over ATM (VTOA) specifications allow adaptation of compressed or uncompressed voice pulse-code modulated (PCM) data streams into streams (virtual circuits) of CBR cells.
An ATM cell, regardless of the traffic it carries, is a packet 53 octets long: 48 octets of payload attached to a 5 octet header. The header contains addressing and management information used to direct the cell from source to destination and to ensure that the negotiated aspects of traffic-flow through the ATM network are met. Each ATM cell is received from a standard OC3c 155.52 MHz fiber optic interface, it takes a 2.83 microseconds to receive a cell. In a PBX that is utilizing standard PCM streams being received from the fiber optic interface, each cell will contain 5.875 milliseconds of voice information assuming a 125 microsecond sampling rate (standard 8000 Hz sampling rate). For CBR traffic, each cell payload consists of a first octet being used as a header to define CBR traffic aspects of the transmission and the remaining 47 octets to carry CBR information such as additional control and data information. Each ATM cell is received by an ATM layer that processes the cell before transferring it on to an ATM adaptation layer 1. The adaptation layer 1 layer processes the ATM cells such that the CBR traffic contained in those cells is presented in a synchronized manner to the PBX. This means that for each call for which cells are being received from the fiber optic interface, a PCM sample as contained in one octet is presented to the PBX every 125 microseconds.
Given the high transfer rate of each ATM cell, there does not initially appear to be a problem in providing the synchronous PCM data to the PBX. However, the fiber optic interface is receiving information from one or more ATM networks and the transmitting end of the information has to assemble the cells based on the 125-microsecond data rate. The end result is that there can be a large variation in cell delay (jitter) as cells are received at the fiber optic interface. This jitter varies due to congestion within an ATM network and delays in the assembly of cells at the transmitting end of the optical fiber. The end result is that there may not be a PCM sample to transmit to the PBX for a particular call. In addition, when wideband transmission is being carried via the ATM cells, a portion of a frame of the wideband data may also not be available to be transmitted to the PBX introducing problems of synchronization of the data of the frame with new frame data after it is received.
The prior art has attempted to resolve the problem of jitter by delaying the transfer of PC samples from the received ATM cells for a predefined period of time (commonly referred to as build out) at the start of each call. A problem with the prior art solution is that the predefined build out period must be as large as any anticipated delay of the receipt of ATM cells by the optical interface. Resulting in unreasonable delays for each call at the very start of the call. In addition, when a delay is encountered in the receipt of ATM cell information that exceeds the predefined build out period, the prior art method simply puts out a predefined value to the PBX until an ATM cell having the necessary call information is received. When the ATM cell is received, the prior art method then determines where within the received information the present instant of time would exist and transfers this information to the PBX. If the delay is long enough for a number of PCM samples to have been replaced with the predefined value, then some of newly received information is discarded since the prior art method will attempt to start in the correct point in real time losing previous samples for earlier times. However, a more serious problem of the prior art is that no change is made in the predefined build out period to take into account the longer delay in the receipt of ATM cells. This is particularly bothersome, since the congestion problems that cause the initial ATM cell to be delayed will most likely occur again since the occurrence was caused by heavy traffic either within an ATM network or a transmitting end of the call. In addition, the predefined build out is defined on a system or individual optical interface basis and will introduce unneeded delay in may calls that are being received by the optical interface which are not subject to the congestion that may be effecting only one of the calls being transmitted via the optical interface.
This invention is directed to solving these and other problems and disadvantages of the prior art. According to the invention, only a nominal predefined build out is defined for each optical interface. If an ATM cell is not received within the proper amount of time for a CBR call from the optical interface, the build out is automatically adjusted so that the build out is equal to the amount of delay that was experienced in receiving the next ATM cell for the call. Advantageously, only the build out for the individual call that actually experienced the delay of the ATM cell is redefined. The other calls being handled by the optical interface are not effected by this automatic adjustment of the build out interval for the individual call. In addition, when an ATM cell is delayed for a particular call, the information that is transmitted to the PBX is the last PCM sample of the present ATM cell for that call. This requires in the case of a wideband call which consists of a frame subdivided into a plurality of channels that the frame must be repeated upon an ATM cell being delayed. Advantageously, when the delayed ATM cell is received, all PCM samples for the call are transmitted to the PBX so that no PCM samples are discarded as is done in the prior art.
These and other features and advantages of the present invention will become more apparent from the following description of an illustrative embodiment of the invention considered together with the drawing.